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Centroid Full Moment Tensor Analysis Reveals Geological and Injection Related Constraints of Induced Seismicity at the Experimental Otaniemi EGS Site, Helsinki Region, Finland

Description: 

In 2018 and 2020 two ~6 km deep geothermal wells in Otaniemi, Helsinki region, Finland were stimulated to develop an experimental enhanced geothermal system. The stimulations activated pre-existing fracture networks and induced thousands of small earthquakes. We present the results of centroid full moment tensor (MT) analysis using the probabilistic seismic source analysis framework Grond and fitting P- and S-waves in the time domain from ~30 stations within 9-km of the drill site. Although the Precambrian shield setting permits using a half space velocity model, channel-wise time shift and amplitude corrections are used to account for small scale crustal variations. The competent medium with low attenuation combined with carefully defined corrections allow the resolution of good MT solutions that reflect the complexities of the source process and source volume.

MT solutions of 301 M_w0.3-2.0 induced events have uniform reverse mechanisms, which is in apparent conflict with the reported regional strike-slip stress field. The average DC component is 80 % and spatially separated DC component clusters reveal small differences in local fracture orientations. The average non-DC component is 20 %, but the non-DC components display clear spatial patterns. In non-DC components a difference is observed between the 2018 and 2020 MT solutions, the 2018 solutions having an average compressive component and the 2020 solutions having an average opening component. Within the 283 events of the 2018 induced seismicity the CLVD component trends towards negative values with increasing distance from injection well. The ISO component over the full set of 301 events shows clear spatial variation that seems not to be related to the positioning relative to the injection wells, but is potentially geologically controlled. We discuss the potential causes of the difference in non-DC components between the two injections including different pre-existing medium and fluid properties in the injection locations, and pore pressure changes following the first stimulation.

Session: Induced Earthquakes: Source Characteristics, Mechanisms, Stress Field Modeling and Hazards [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Annukka

Student Presenter: Yes

Invited Presentation: 

Authors